The present invention relates to a rolling bearing for use in vehicles, agricultural machines, construction machines, steel making machines and other industrial machines. More particularly, the present invention relates to a rolling bearing having sufficiently improved wear resistance so that it is suitable for use under a severe condition (such as a high temperature environment, a case where the rolling contact occurs with slippage accompanying, or the like).
Rolling bearings are used under extremely severe conditions such that they are subjected to repeated shearing stress under high contact pressure. In order to withstand the applied shearing stress to thereby secure the necessary rolling fatigue life (hereinafter also referred to simply as "life"), conventional rolling bearings have been made of a high-carbon chromic bearing steel (SUJ 2), which is hardened and tempered to provide the Rockwell hardness of H.sub.R C 58 to 64.
Case hardening steels have also been used to extend the life. In order to set a hardness curve in accordance with the distribution of internal shearing stresses due to a contact pressure, low carbon case hardening steels such as SCR 420H, SCM 420H, SAE 8620H, SAE 4320H and the like, which have the superior hardenability, are carburized or carbonitrided, followed by hardening and tempering to produce bearing components that have the surface hardness of H.sub.R C 58 to 64 and the core hardness of H.sub.R C 30 to 48. Thus, the required service life has been secured by the above heat treatments.
Recently, since it tends to require machines with higher load and speed performances in which the rolling bearings are employed, the conditions for the use of bearings become extremely severe. Under the circumstances, there have been caused several problems.
A rising temperature occurs because of the advancement of the higher load or speed performance so that the hardness of the bearing components is reduced to cause deterioration in the rolling fatigue and wear characteristics. In particular, the wear resistance is extremely deteriorated by the increase of slippage due to the higher speed and insufficient lubrication due to the decrease in the viscosity of lubricant under the rising temperature, so that the life of the bearing is reduced.
To deal with these problems, for example, Unexamined Japanese Patent Publication No. Hei. 4-165045 proposes that a bearing steel base is added with Si and Mo as elements improving a resisting property for temper softening in order to increase the strength at a high temperature, so that a bearing which is produced with the base has an extending life under the high temperature.
Further, Unexamined Japanese Patent Publication No. Sho. 64-83625 has proposed that a Si-containing steel is subjected to a so-called "marstressing" treatment in which nitrogen is introduced into the steel surface by supplying ammonia gas into a hardening atmosphere, and subsequently tempering the steel at a high temperature of 400.degree. C. so that it maintains a hardness of at least H.sub.R C 60 to secure the extending life under the high temperature.
It is known that the wear resistance of steels is improved if fine carbides are precipitated on their surface. An approach has been made to improve the wear resistance of bearing materials by adding a large amount of carbide-forming elements such as Cr, Mo and V so that a large amount of carbides are precipitated on the bearing's surface layer to such a degree that it does not adversely affect the life of the bearing. An example based on the approach has been proposed to employ a high-speed steel M50 or the like as a heat-resistant bearing material in which the precipitation hardening is caused by hardening an alloy steel including a large amount of Cr, Mo and V contents, followed by tempering at a high temperature.
The conventional methods described in Unexamined Japanese Patent Publication Nos. Hei. 4-165045 and Sho. 64-83625 are both effective in improving the high-temperature strength of bearings so that their rolling fatigue characteristics are improved. However, there is a problem which it is not very effective in achieving significant improvements of wear resistance.
The other conventional method also has caused the following problems in which the method employs a precipitation hardening steel M50 as a heat-resistant bearing material. First, due to the high C concentration, macro carbides of Cr, Mo and V already occur in a row material so as to reduce the workability in pretreatments. Secondly, the macro carbides of Cr, Mo and V cause stress concentration around themselves because of the occurrence, where it serves as a starting point of flaking contrary to decrease the life of the bearing. In order to solve these problems, it is necessary to perform a solution treatment at a high temperature (e.g. 1,100.degree. C.) so that the macro carbides are solved into the matrix, followed by a special heat treatment to make the carbides fine-grained. However, this method has another problem of not only requiring an equipment for carrying out the special heat treatment but also reducing its productivity.